1. Field of the Invention
The present invention relates to an uninterruptible power supply (UPS) and more particularly to a UPS with dual chargers and sinusoidal output.
2. Description of the Related Art
With reference to FIG. 4, a conventional UPS has an output filter 71, a charger 72, a transfer switch 73, a DC-to-DC controller 74, a battery pack 75, a DC-to-DC conversion module 76 and an inverter 77. An input terminal of the output filter 71 is connected to an AC mains. The charger 72 and the transfer switch 73 are connected to an output terminal of the output filter 71. The transfer switch 73 has a first contact, a second contact and a common contact. The second contact is connected to the output terminal of the output filter 71. The common contact serves as a power output terminal connected to a load. When the mains power is normal, the charger 72 is controlled by the DC-to-DC controller 74 to charge the battery pack 75. An input terminal of the DC-to-DC conversion module 76 is connected to the battery pack 75. An input terminal of the inverter 77 is connected to the output terminal of the DC-to-DC conversion module 76, and an output terminal of the inverter 77 is connected to the second contact of the transfer switch 73.
When the mains power is normal, the mains power passes through the output filter 71, the first contact and the common contact of the transfer switch 73 to supply power to the load. When the mains power is abnormal, the transfer switch is switched to disconnect the first contact from the common contact, and to connect the second contact with the common contact so as to enter a battery mode. Under the battery mode, the DC-to-DC conversion module 76 boosts the DC voltage of the battery pack 75 and outputs the DC voltage to the inverter 77 for the inverter 77 to convert the DC voltage into an AC voltage and supply power to the load through the transfer switch 73.
Although the foregoing off-line UPS can supply backup power when the mains power is abnormal, its application is limited to the resistive loads or current loads because of its square wave output. In the case of inductive loads or mixed loads, the load equipment connected to the off-line UPS may be damaged. On the other hand, as the output of on-line UPSs has a sinusoidal waveform identical to that of the mains power, the on-line UPSs are applicable to inductive loads, purely capacitive loads, and mixed loads. In this regard, the applicant files a China Patent Application CN 20120138935.7, entitled “Method for controlling output waveform of uninterruptible power supplies”, which discloses a UPS providing multiple output waveforms for users to selectively switch to one of the output waveforms of the UPS after the UPS enters a battery mode. The output waveforms include a square waveform and a sinusoidal waveform. Accordingly, the output waveforms can be selectable based on the type of load, thereby avoiding damage to the load equipment and enhancing the operation efficiency.
Despite the feasibility of sinusoidal output, if the on-line and off-line UPSs are practically applied to inductive loads, capacitive loads or mixed loads having energy storage elements, those energy storage elements discharge its stored energy in the form of voltage or current to the DC-to-DC conversion modules of the UPSs after the loads are switched off. The energy discharged by the energy storage elements after the loads are switched off is called a “phantom power”. Even when the DC-to-DC conversion modules are not operating, the “phantom power” is stored in the circuit loops of the DC-to-DC conversion modules. After the DC-to-DC conversion modules are operating, the “phantom power” is then added to pulsating DC voltage outputted from the DC-to-DC conversion modules to further distort the sinusoidal waves outputted from the inverters. The current approach used to tackle such issue is to consume the “phantom power” by converting it into heat and dissipating the heat. However, such approach results in temperature rise of the UPS and energy waste.